This invention relates to a heat-fixing method of fixing an image on a recording medium by heating, which is employed in an image recording apparatus such as, for example, a Xerographic copying machine wherein a Xerographic system is utilized to record an image onto a recording medium with a toner.
Generally, a recording medium, on which information is recorded by an image recording apparatus, has the following properties. That is, the smoother the surface condition of the recording medium after fixing, the higher the reflection optical density. Accordingly, the recording quality is good, and the fixing strength is also enhanced.
As an example of such recording medium, there is one employed in a Xerographic copying machine utilizing a Xerographic system, wherein an image is recorded on the recording medium with a toner.
The Xerographic copying machine comprises a photoconductive drum whose outer peripheral surface is formed of electrostatic material into a photoconductor. The arrangement is such that the photoconductor on the surface of the photoconductive drum is charged uniformly and is then exposed to light to form a latent image. Subsequently, toner is applied to the latent image to develop the same, i.e., to form a toner image. The toner image is then transferred to the recording medium and is fixed by a fixing unit. Thereafter, the recording medium is discharged out of the copying machine.
Various methods have been proposed as a fixing method of fixing the toner image on the recording medium to record the image onto the same. In this connection, the mainstream is a method of heating the toner image to melt the same, thereby fixing the image onto the recording medium. As a fixing system on the basis of such heating and melting, there have conventionally been known various systems including an oven system, a radiant system and the like. Generally, however, a heat-roller system has widely been used, which is high in thermal efficiency and which is capable of raising a processing speed.
The heat-roller system comprises a pair of heat roller and pressure or backup roller which are so arranged as to have their respective axes extending in parallel relation to each other in a common plane. The heat roller has a peripheral surface heated to a predetermined temperature by a heat source such as a halogen lamp or the like incorporated in the heat roller. The backup roller has an outer peripheral surface which is formed of resilient or elastic material having heat resistance. The backup roller is in pressure contact with the heat roller with a predetermined pressure. The recording medium having carried thereon an unfixed toner image is caused to pass through a nip between both the rollers. The toner image is melted by the heat from the heat roller and is fused onto the recording medium. By this melting due to heat, the toner surface is smoothed so that there is obtained a high recording quality and a high fixing strength.
Furthermore, an image recording apparatus is also known which employs a photo and pressure sensitive recording medium including a developer sheet. In this apparatus, a developer agent and a binder agent coated on the developer sheet are melted and softened to smooth the surface of the developer sheet, thereby giving a gloss to enhance the recording quality. The apparatus also utilizes a heat-roller system similar to that used in fixture of the toner mentioned above.
In the system in which the recording medium is heated by the heat roller to melt and fuse or to fix the image onto the recording medium, however, there may be a case where the toner serving as image forming material, and the developer agent and the binder agent serving as image forming material are adhered to the heat roller to cause a so-called offset phenomenon, so that the fixing cannot be made excellently and no gloss can be obtained. On the contrary, the image is disfigured. Furthermore, if the temperature of the heat roller is high excessively, for example, above 120.degree. C., the image forming material is fused and adhere to the heat roller, making it impossible for the recording medium to pass through the nip between the rollers. On the other hand, if the temperature of the heat roller is low excessively, for example, below 120.degree. C., the image forming material is prevented from being fused and adhering to the heat roller, but there arises such a problem that no sufficient gloss can be obtained. In this manner, it is not easy to regulate the temperature of the heat roller and the feed speed of the recording medium, making it extremely difficult to realize the excellent fixing high in recording quality.